A. Single Column Ion Chromatography
Single Column Ion Chromatography (SCIC) is a method of ion analysis in which ions are separated in an ion exchange column (e.g., separator column) and subsequently measured by a conductivity detector connected directly to the separator column. In SCIC, special ion exchange resins of low capacity, and eluants with either much higher or much lower equivalent conductance than the ions being measured must be employed. In ion chromatography, sample ions generate a signal at a conductivity detector. The signal is proportional to the sample ion concentration and is the difference in equivalent conductance between the sample ion and the eluant ion. SCIC sensitivity is limited by the difference in equivalent conductance between the sample ions and the eluant ions. This sensitivity is adequate and even preferred for some sample types, especially for cationic samples, where the difference in equivalent conductance between the sample and eluant ions is very large. However, for many other samples, particularly anionic samples, where the difference in equivalent conductance between the sample ions and eluant ions is small, sensitivity can be greatly increased by a second and preferred type of ion analysis called chemically suppressed ion chromatography (SIC).
B. Suppressed Ion Chromatography (SIC)
Suppressed ion chromatography (SIC) is a form of commonly practiced ion analysis characterized by the use of two ion-exchange columns in series followed by a flow through conductivity detector. The first column, called the separation column, separates the ions of an injected sample by elution of the sample through the column using an electrolyte as an eluant, i.e., usually dilute base or acid in deionized water. The second column, called the “suppressor” or “stripper”, serves two purposes. First, it lowers the background conductance of the eluant to reduce noise. Second, it enhances the overall conductance of the sample ions. The combination of these two factors significantly enhances the signal to noise ratio, thus increasing sensitivity.
This technique is described in more detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559. In addition, suitable ion exchange packings for the separation column are described in detail in U.S. Pat. Nos. 3,966,596, 4,101,460 and 4,119,580. A detailed description of ion chromatography is additionally provided in Small et al., “Proceedings of an International Conference on the Theory and Practice of Ion Exchange,” University of Cambridge, U.K., July, 1976; and also, Small et al., “Novel Ion Exchange Chromatographic Method Using Conductimetric Detection”, Analytical Chemistry, Vol. 47, No. 11, September 1975, pp. 1801 et seq. The foregoing patents and literature publications are fully incorporated herein by reference.
C. Gradient Elution Technology
To separate or elute sample ions retained on an ion-exchange column, an eluant containing co-ions of the same charge of the sample ions is routed through the separation column. The sample co-ions in the eluant partially displace the sample ions on the ion-exchange column, which cause the displaced sample ions to flow down the column along with the eluant. Typically, a dilute acid or base solution in deionized water is used as the eluant. The eluant is typically prepared in advance and routed through the column by either gravity or a pump.
Rather than using a homogenous eluant throughout the separation process, it is sometimes advantageous to use a gradient eluant, i.e., an eluant wherein the concentration of one or more components changes with time. Typically, the eluant starts at a weak eluting strength (e.g. a low concentration of the sample co-ions) and gets stronger (e.g. a higher concentration of the sample co-ions) during the separation process. In this way, easily eluted ions are separated during the weaker portion of the gradient, and ions that are more difficult to elute are separated during the stronger portion of the gradient. The eluant concentration changes during the gradient and suppressing or balancing the concurrent change in background conductance is required so the sample signal may be discriminated from the background signal. An example of such gradient elution techniques are disclosed in U.S. Pat. Nos. 4,751,189 and 5,132,018, the entire disclosures of which are incorporated herein by reference.
While the above patents utilize solutions prepared in advance to form a gradient eluant, U.S. Pat. No. 5,045,204 to Dasgupta et al. uses electrochemical methods to generate a high purity eluant stream that may flow directly to the separation column as it is produced, and which may be generated as a gradient. In the Dasgupta patent, a product channel is defined by two permselective membranes and is fed by a source of purified water. One of the permselective membranes only allows the passage of negatively charged hydroxide ions, which are generated on the side of this membrane opposite the product channel by the electrolysis of water at a cathode. The hydroxide ions are driven by an electric field through the membrane into the product channel in an amount corresponding to the strength of the electric field. The other permselective membrane only allows the passage of positively charged ions. On the side of this membrane opposite the product channel there is a source channel, which is continuously fed with a NaOH solution and in which an anode is positioned. The Na+ ions are driven by the electric field through the membrane into the product channel in an amount corresponding to the strength of the electric field. By this process, a high purity sodium hydroxide (NaOH) solution is produced. This solution may be used as the eluant for a chromatography column, and the concentration of this eluant may be varied during the chromatographic separation by varying the strength of the electric field, thereby generating a gradient eluant.
The foregoing methods of elution ion chromatography suffer from certain disadvantages, however. Among these disadvantages is that an outside source of eluant or eluant counter-ions is required. Also, after eluting the sample ions from the chromatography column, all of these eluants require suppression in order to provide an accurate quantitative analysis of the sample ions. Finally, in general practice, all of the above methods of eluting are only applicable to one of either cation or anion sample ions within a single sample run. If one wishes to analyze both the cations and anions from a single sample, two chromatographic separations must be performed using either two apparatuses and two distinct eluants, or a single instrument with two or more columns and complex switching valves.
D. Prior Suppressor Technology
Chemical suppression for IC serves two purposes. First, it lowers the background conductance of the eluant to reduce baseline noise. Second, it enhances the overall conductance of the sample ions to increase the signal. The combination of these two factors significantly enhances the signal-to-noise ratio, and increase the detectivity of the sample ions. For example, in anion analysis, two ion-exchange reactions take place in a suppressor column when the eluant comprises sodium hydroxide and the ion exchange packing material in the suppressor column comprises exchangeable hydronium ions:    1) Eluant: NaOH+Resin-SO3−H+---->Resin-SO3−Na++H2O    2) Analyte: NaX+Resin-SO3−H+---->Resin-SO3−Na++HX            where X=anions (Cl−, NO2−, Br−, etc.)        
The relatively high conductivity sodium hydroxide eluant is converted to the relatively low conductivity water when the sodium ions from the eluant displace the hydronium ions on the ion exchange packing material in the suppressor. The sample anions are converted from their salt form into their more conductive acid form by exchanging their counter-ions for hydronium ions in the suppressor. The eluant is preferably a solution of any salt that forms a weakly conductive acid after going through the suppressor. Examples of such eluants in anion analysis include sodium hydroxide, sodium carbonate, or sodium tetraborate solutions.
Various suppressor devices that operate on the above principles have been used for IC. These include:
1. Packed-Bed Suppressors
Packed-Bed Suppressors were introduced in about 1973 (see, for example, U.S. Pat. Nos. 3,918,906, 3,925,019, 3,920,397, 3,926,559, 4,265,634, and 4,314,823, the entire disclosures of which are incorporated herein by reference). These suppressors consist of large columns containing strong acid cation-exchange resins in hydronium form (for anion analysis). In order to house enough resin, these columns are very large (i.e., 250 mm×7.8 mm). However, these columns have a large dead volume, which causes considerable peak dispersion and broadening. This, in turn, results in a loss of chromatographic efficiency. Moreover, after several hours of operation, the resin bed becomes exhausted (all the hydronium ions on the exchange sites are replaced by the sample and the eluant counterions). The suppressor column must then be taken off-line and regenerated by flushing the column with an acid to regenerate the hydronium ion exchange sites in the resin bed. The regeneration of the suppressor column, of course, is time consuming and interrupts the analysis.
Another disadvantage of these packed bed suppressors is that weakly ionized species such as organic acids can penetrate the protonated cation exchange sites and interact by inclusion within the resin bed. This causes variable retention times and peak areas as the suppressor becomes exhausted. Also, some ions can undergo chemical reactions in the suppressor. For example, nitrite has been shown to undergo oxidation in these prior art packed bed suppressors leading, to variable recovery and poor analytical precision.
2. Hollow-Fiber Membrane Suppressors
In about 1982, hollow fiber membrane suppressors were introduced (see, for example, U.S. Pat. Nos. 4,474,664 and 4,455,233, the entire disclosures of which are incorporated herein by reference). Hollow fiber membrane suppressors were designed to overcome the drawbacks of the packed bed suppressors. The hollow fiber membrane suppressors consist of a long, hollow fiber made of semi-permeable, ion-exchange material. Eluant passes through the hollow center of the fiber, while a regenerating solution bathes the outside of the fiber. Suppressor ions cross the semi-permeable membrane into the hollow center of the fiber, and suppress the eluant. The regenerating solution provides a steady source of suppressor ions, allowing continual replacement of the suppressor ions as they pass to the eluant flow channel in the hollow center of the fiber. The main advantage of the hollow fiber design is that the chromatography system can be continuously operated because there is no need to take the suppressor off-line for regeneration, as is the case with the packed bed suppressors.
However, the hollow fiber design introduced new problems. The small internal diameter of the fibers reduces the surface area available for ion exchange between eluant and the regenerant. This limits the suppression capability of the hollow fiber suppressors to low flow rates and low eluant concentrations. Additionally, because the fiber is bathed in the regenerant solution, the counterion of the suppressor ions can leak into the eluant channel, and cause higher background conductivity and baseline noise at the detector.
3. Flat-Sheet Membrane Suppressors
Flat-sheet membrane suppressors were introduced in about 1985 (see, for example, U.S. Pat. Nos. 4,751,189 and 4,999,098, the entire disclosures of which are incorporated herein by reference). In these suppressors, the ion exchange tubing in the hollow-fiber suppressor is replaced with two flat semi-permeable ion exchange membranes sandwiched in between three sets of screens. The eluant passes through a central chamber which has ion exchange membrane sheets as the upper and lower surfaces. The volume of the eluant chamber is very small, so band broadening is minimal. Since the membrane is flat, the surface area available for exchange between the sample counterions and the suppressor ions in the regenerant is greatly increased. This increases the suppression capacity allowing high flow rates, high eluant concentration, and gradient analyses. Preferably, the regenerant flows in a direction counter to the sample ions over the outer surfaces of both membranes, providing a constant supply of suppressor ions.
A major drawback, however, of membrane suppressors is that they require a constant flow of regenerant to provide continuous suppression/operation. This consumes large volumes of regenerant and produces large volumes of chemical waste, significantly increasing operating cost. An additional pump or device is required to continuously pass the regenerant through the suppressor, increasing the instrument's complexity and cost while reducing reliability. Also, organic compounds can irreversibly adsorb onto the hydrophobic ion-exchange membrane, reducing its efficiency to the point where it requires replacement (membranes are typically replaced every six months to two years). Finally, the membranes are very thin and will not tolerate much backpressure. Thus, membrane rupture is a concern anytime downstream backpressure increases due to blockages.
4. Solid Phase Chemical Suppressor (SPCS)
Alltech Inc., the assignee of the present application, developed solid phase chemical suppressors (SPCS) in about 1993, which were essentially an improved version of the original packed bed suppressors. Problems associated with the original packed bed suppressors, such as band broadening, variable retention time and peak area, and the oxidation of nitrite in the suppressor, were greatly reduced. The Alltech SPCS uses disposable cartridges containing ion exchange packing material comprising suppressor ions as the suppressor device. The inexpensive cartridges are simply discarded and replaced with a new cartridge when the suppressor ions are exhausted. Thus, no regeneration is required, thereby eliminating the need for expensive or complex systems for regenerating suppressor ions.
In Alltech's SPCS system, a 10-port switching valve and two disposable suppressor cartridges are typically employed. The effluent from the analytical column flows through one cartridge at a time. While one cartridge is being used, the suppressed detector effluent (typically water or carbonic acid) flows through the other suppressor cartridge to pre-equilibrate the cartridge. This reduces the baseline shift due to conductance change when the valve is switched to the other suppressor cartridge. When all the suppressor ions from one cartridge are replaced by the eluant and sample counterions, the valve is switched, placing the second cartridge in the active position, and the exhausted suppressor cartridge is replaced. This allows continuous operation. However, the Alltech system still requires someone to switch the valve manually when the first cartridge is exhausted. Each cartridge typically provides between 6 to 9 hours of operation, and thus fully unattended or overnight operation might not be possible in certain applications with the Alltech SPCS system.
5. Electrochemical Suppression
Electrochemical suppressors were introduced in about 1993. These suppressors combine electrodialysis and electrolysis in a flat-sheet membrane suppressor column similar to those described under heading section 3 above (see U.S. Pat. Nos. 4,459,357 and 5,248,426, the entire disclosures of which are incorporated herein by reference).
For example, U.S. Pat. No. 5,248,426 to Stillian et al. discloses a suppressor which contains a central chromatography effluent flow channel bordered on both sides by ion exchange membranes with exchangeable ions of the opposite charge of the sample ions. On the side of each membrane opposite the effluent flow channel are first and second detector effluent flow channels. The sample ions and eluant are routed through the chromatography effluent flow channel, and the water-containing detector effluent is routed through the detector effluent flow channels in the suppressor. An electrode is positioned in both of the detector effluent flow channels.
By energizing the electrodes, an electrical potential is generated in the suppressor transverse to the liquid flow through the chromatography flow channel. When the water-containing detector effluent contacts the energized electrodes, it undergoes electrolysis. In anion analysis for example, the suppressor hydronium ions generated at the anode in a first detector-effluent channel are transported across the ion exchange membrane into the chromatography effluent flow channel, where they combine with the sample anions to form the highly conductive acids of the sample anions. The suppressor hydronium ions also combine with the hydroxide ions in the eluant (in anion analysis) to convert the eluant into the relatively non-conductive water. At the same time, the eluant and sample counterions are transported from the chromatography effluent channel across the ion exchange membrane into a second detector effluent flow channel where they combine with the hydroxide ions generated by the electrolysis of the water-containing detector effluent at the cathode in the second detector effluent flow channel. The resulting bases of the eluant counterions are then routed to waste.
Thus, the electric field generated in the suppressor column disclosed in Stillian et al. simultaneously generates suppressor ions and promotes ion-flow between the electrodes in a direction transverse to the fluid flow through the suppressor. The mass transport of ions is across a first ion exchange membrane from a first detector effluent flow channel to the chromatography effluent flow channel, and across a second ion exchange membrane to a second detector effluent flow channel.
Although the electrochemical suppressor device disclosed in Stillian et al. offers certain advantages (i.e. no separate regenerant source is required), it still suffers from certain disadvantages. Irreversible adsorption of organic components and membrane breakage under pressure may still occur in the apparatus and method disclosed in Stillian. Also, the method of electrochemical suppression disclosed in Stillian can only be used to analyze solely anions or solely cations in any one sample. Finally, the Stillian method does not work well with electroactive eluants or organic solvents. Electroactive eluants, such as hydrochloric acid, commonly employed as an eluant for cation analysis, undergo electrochemical reaction in the suppressor producing by-products that damage the membrane. Also, certain organic eluant components such as methanol undergo electrochemical reaction in the electrochemical suppressor producing by-products that are conductive and which interfere with the detection of sample ions. Such electroactive eluant systems may not be effectively employed in the Stillian method.
The column, apparatuses, and methods of the present invention reduce or avoid many of the foregoing problems.